This project responds to RFA-DA-001. Its objective is to extend the utility of positron emission tomography (PET) flurobenzyltrozamicol (FBT) radioligand in the study of brain to the study of aging skeletal muscle. Growing evidence supports a role for neural degeneration in the chronic, age-related structural and functional decline of skeletal muscle. It will test the hypothesis that the noninvasive imaging procedure FBT-PET can be used to assess skeletal muscle innervation throughout the aging process in rodents. Results will inform an analysis of its role in human sarcopenia. The following specific aims will be analyzed in young adult (7-month) and aged (24-month) mice fed ad libitum and calorie-restricted, a maneuver known to retard motor neuron loss with aging. Specific Aim 1. To quantify hindlimb muscle innervation using FBT-PET. Time/activity curves in lower hindlimb muscles and differences in FBT uptake between three experimental groups will be measured. Loss of muscle volume with aging will be analyzed by MRI. FBT-PET and MRI signals will be co-registered. Signal specificity will be analyzed by measuring: (a) FBT vascular clearance, (b) [3H]FBT binding, (c) [3H]FBTautoradiography, (d) blocking of 18F-FBT uptake with cold FBT/vesamicol, and (e) FBT uptake by VAChT knock-down mice. Specific Aim 2. To examine the relationship between FBT-PET and mouse skeletal muscle function in vivo and in vitro using a well-characterized functional assessment battery and single muscle force measurement, respectively. These studies will determine a relationship between age-related decrease in muscle function and the magnitude and extension of muscle denervation. Specific Aim 3. To quantify mouse muscle VAChT density using in vitro determinations and to establish a relationship with FBT-PET. Immunohistochemistry (IH) for VAChT will allow us to measure the transporter's density in multiple sections of longitudinal muscle. IH signal specificity will be determined by double staining with labeled a-bungarotoxin, which stains the post-terminal AChR. PUBLIC HEALTH RELEVANCE: The contribution of muscle denervation to sarcopenia remains elusive, particularly in humans. Validation of FBT (flurobenzyltrozamicol)-Positron Emission Tomography (PET) as a novel, accurate, noninvasive measure of skeletal muscle innervation in rodents will provide valuable data that can be rapidly translated into an effective research tool in animals and humans. Clinically, it can be used to evaluate interventions aimed at preventing and/or ameliorating the contribution of muscle denervation to sarcopenia and subsequent physical disability in the elderly. Results of this project are intended to inform the preparation and submission of applications necessary to gain regulatory approval to examine skeletal muscle denervation and its influence on sarcopenia in humans. In a second phase, distribution of the radiotracer FBT to other institutions will allow a multi-center evaluation of its utility in quantifying muscle denervation associated with human sarcopenia.